1. Field of the Invention
The present invention relates to an optical system and, in particular though not exclusively, to an optical system that can be used for photographing optical systems, observation optical systems, projection optical systems, and readout optical systems.
2. Description of the Related Art
Retro focus lens systems (negative lead type) have been widely used to provide a wide angle (wide viewing angle) to lens systems. Such a retro focus lens system includes a lens component with negative refractive power disposed in the front and a lens component with positive refractive power disposed in the rear, thereby providing a short focal length and a long back focus. As used herein, the term “front” refers to a side adjacent to a subject in photographing optical systems (e.g., cameras) and a side adjacent to a screen in projection optical systems (e.g., projectors). The term “rear” refers to a side adjacent to an image plane in the photographing optical system and a side adjacent to an original image in the projection optical system. In correcting and/or reducing aberration of a lens, the retro focus lenses have a disadvantage in that negative distortion (barrel distortion) tends to occur, since a lens component with negative refractive power is disposed in the front and the refractive power is not symmetrical. By forming a negative lens in the lens component with negative refractive power from a material of a high index of refraction, the negative distortion can be reduced. However, in general, since materials of a high index of refraction are of high dispersion, negative chromatic aberration of magnification (transverse chromatic aberration) tends to occur.
To correct and/or reduce the negative chromatic aberration of magnification in a retro focus lens, a method is known in which a positive lens formed from a low dispersion material of extraordinary partial dispersion (e.g., fluorite) can be used in a lens component disposed at the rear of an aperture stop and having an equivalently large height (distance from an optical axis) H of entering paraxial chief ray on the lens surface. In many conventional retro focus optical systems, the chromatic aberration is reduced using this method. A variety of retro focus optical systems using this method has been discussed (refer to, for example, Japanese Patent Laid-Open No. 06-082689 and Japanese Patent Laid-Open No. 2002-287031).
Additionally, a method for correcting and/or reducing chromatic aberration using a diffractive optical element without using a material of extraordinary partial dispersion is discussed in, for example, Japanese Patent Laid-Open No. 2000-147373 and Japanese Patent Laid-Open No. 2002-156582 (corresponding to U.S. Pat. No. 6,496,310). In these patent documents, a retro focus optical systems having equivalently well corrected chromatic aberration are proposed by appropriately combining a diffractive optical element with a refractive optical element.
Among optical materials having a chromatic aberration correcting and/or reducing function related to the optical characteristic of a diffractive optical element, a liquid material, which can have a characteristic of equivalently high dispersion and extraordinary partial dispersion, is known. Achromatic optical systems using that material are discussed in, for example, U.S. Pat. Nos. 5,731,907 and 5,638,215.
In general, a long total length of an optical system (total length of lenses) can correct the chromatic aberration equivalently well. If the total optical length is reduced, a large amount of chromatic aberration appears.
This is because the method for correcting and/or reducing chromatic aberration employs low dispersion and extraordinary partial dispersion that a material such as fluorite possesses to reduce chromatic aberration generated by the front element itself. When correcting and/or reducing chromatic aberration generated when the length of a lens is reduced, for example, in an optical system that employs a low-dispersion material, which can have a large Abbe number (e.g., fluorite), the chromatic aberration does not change unless the power on the lens surface is largely changed. Accordingly, it can be difficult to correct and/or reduce the chromatic aberration and other types of aberration (e.g., spherical aberration, coma aberration, and astigmatism) contemporaneously.
In contrast, a diffractive optical element has a sufficient correcting function of chromatic aberration. However, optical systems including a diffractive optical element can degrade a focusing performance since unwanted diffracting light having diffracting orders other than the designed diffracting order becomes color flair light. Some optical systems including a diffractive optical element concentrate energy on the designed diffracting order by using a so-called layered diffractive optical element so as to largely reduce unwanted diffracting light. However, when photographing a subject having high brightness, the diffraction flair might still appear.
Materials described in U.S. Pat. Nos. 5,731,907 and 5,638,215 are liquid. Therefore, the characteristic of the index of refraction and the characteristic of dispersion are largely changed in accordance with the change in temperature. Thus, the resistance to the surrounding environment is not sufficient.